Those actively involved in audio or audio-visual industries have continually strived to overcome the imperfections of reproduced sound. Presently, with the onslaught of interactive multimedia computer systems, and other audio-visual advances, the concern over audio quality has heightened. Consequently, there are renewed efforts among the audio industry to develop technological improvements in sound recordings and their reproduction.
Imperfections of reproduced sound can result from, among other things, microphones which ineffectively record sound, and speakers which ineffectively reproduce recorded sound. Attempts at sound image enhancement by those in the relevant industries have resulted in methods which record and encode the positional information of a sound's origin along with the sound information itself. Such methods include the multi-channel surround systems which operate using specially encoded audio information, and special decoding systems to interpret the information.
Sound enhancement systems which do not require specially recorded sound are typically less complex and much less expensive. Such systems include those which introduce unnatural time-delays or phase-shifts between left and right signal sources. Many of these systems attempt to compensate for the inability of a microphone to mimic the frequency response of a human ear. These systems may also attempt to compensate for the fact that, because of the location of a speaker, the perceived direction of sound emanating from that speaker may be inconsistent with the original location of the sound. Although the foregoing systems attempt to reproduce sound in a more realistic and life-like manner, use of such methods have resulted in mixed results in the competitive audio enhancement field.
Other sound enhancement techniques operate on what are termed sum and difference signals. The sum and difference signals represent the sum of left and right stereo signals, and the difference between left and right stereo signals, respectively.
It is known that boosting the level of difference signal in a pair of stereo left and right signals can widen a perceived sound image projected from a pair of loudspeakers, or other electroacoustic transducers, placed in front of a listener. The widened sound image results from amplification of ambient or reverberant sounds which are present in the difference signal. This ambient sound is readily perceived in a live sound stage at the appropriate level. In a recorded performance, however, the ambient sounds are masked by the direct sounds, and are not perceived at the same level as a live performance.
There have been many attempts to improve ambient sound information from a recorded performance by indiscriminately increasing the difference signal over a broad frequency spectrum. An indiscriminate increase in the difference signal, however, can undesirably affect a person's sound perception. For example, boosting of the difference signal in the mid-range of audio frequencies can lead to sound perception which is overly sensitive to the position of a listener's head.
A critically-acclaimed sound enhancement technique which processes the sum and difference signals is disclosed in U.S. Pat. Nos. 4,748,669 and 4,866,774 both issued to Arnold Klayman, the same inventor for the invention disclosed in the present application.
As disclosed in both the '669 and the '774 patents, a sound enhancement system provides either dynamic or fixed equalization of the difference signal in selected frequency bands. In such a system, equalization of the difference signal is provided to boost the difference signal components of lower intensity without overemphasizing the stronger difference signal components. The stronger difference signal components are typically found in a mid-range of frequencies of approximately 1 to 4 KHz. These same mid-range of frequencies correspond to those which the human ear has heightened sensitivity. The various embodiments of the systems disclosed in the '669 and '774 patents also equalize the relative amplitudes of the sum signal in specific frequency bands to prevent the sum signal from being overwhelmed by the difference signal. Moreover, the level of difference-signal boost provided by the '669 and '774 enhancement systems is a function of the sum signal itself.
The specific advantages of selectively boosting the sum and difference signals in light of the human auditory response characteristics, is fully disclosed in detail in U.S. Pat. No. 4,748,669 and U.S. Pat. No. 4,866,774.
Even with the foregoing audio enhancement techniques, there is a need for an audio enhancement system that can provide high quality stereo image enhancement and which can meet all of the demands of the burgeoning computer multimedia market, and those of the audio and audio-visual markets in general. The stereo enhancement system disclosed herein fulfills this need.